1. Field of the Invention
The present invention relates to a multi-chip package semiconductor device in which a drive chip having an analog circuit and a logic chip having a digital circuit are mounted within the same package.
2. Description of the Related Art
A digital circuit that performs logical processing of digital data is usually voltage-driven and is composed of many small transistors that are on-off driven. It is difficult to incorporate a power-supply circuit having current driving capabilities sufficient for driving a digital circuit into a digital-circuit chip.
For this reason, in the case of a logic chip in which digital circuits are integrated, a power supply necessary for the logic chip is often fabricated as a power-supply circuit by using bipolar transistors formed from a different chip to that of the logic chip.
For example, Japanese Patent Laid-Open Publication No. 2002-57270 discloses a semiconductor device having a plurality of chips with different power supplies. This semiconductor device is such that in each of the chips, power is received from separate power supplies through power-supply circuits that are not diagrammatically shown.
Examples of documents describing the related art include Japanese Patent Laid-Open Publication No. Hei 7-23277 and Japanese Patent Laid-Open Publication No. Hei 11-187308.
A logic circuit performs various kinds of logical operations on the basis of a clock of relatively high frequency. Therefore, these operations basically involve turning on and off transistors that are connected to a power line and a ground line, thereby setting a signal line at H-level or L-level. Therefore, switching noise is apt to be superposed on signals of the power line and ground line of a logic circuit. On the other hand, an analogue circuit is provided with an operational amplifier that amplifies detection signals of a hole sensor, a gyro sensor and the like. These detection signals are very small signals, and hence the effect of noise should be eliminated as much as possible. That is, if switching noise is superposed on signals of the power line and ground line, the effect of this switching noise manifests itself in the output of the operational amplifier, and amplified signals of the noise are included in the amplified signals of the micro detection signals.